This invention is directed to particle counting apparatuses which provide a statistic correction to a detected train of particle derived count pulses, such that effective random coincidence inaccuracies of count do not induce ultimate counting error.
The particle counting apparatuses concerned employ particle sensing zones in which more than one particle might reside at any one time and thereby randomly generate a coincidence condition. This invention particularly is directed to, but not limited to the determination of nonelectrical properties, such as size and count of microscopic particles, by measuring electrical properties (Patent Office class 324-71NE).
Now well known in the art of electronic particle counting and analyzing is apparatus marketed primarily under the trademark "Coulter Counter". Such apparatus and portions thereof are disclosed in several U.S. Pat. Nos., for example 2,656,508; 2,985,830; and 3,259,842 (each in class 324-71). A significantly important portion of such Coulter type of apparatus is the minute scanning aperture or scanning ambit or sensing zone relative to or through which are to pass and be detected single particles at a rate often well in excess of one thousand per second. Because of the physical parameters of the scanning aperture, and particle concentration, there frequently results the coincidence of two particles in the scanning ambit. As a result, there is effectively detected and counted only one particle, not two.
Although such primary form of coincidence loss of count is random in time, it follows a statistically ascertainable form from which curves, tables, and formulae are obtainable. A relatively simple one of such formulae is: N' = K N.sup.2 in which N'= the total number of coincidences, i.e., the required addend; k = a constant which relates primarily to the physical parameters of the scanning elements of the apparatus and N = the detected number of particles, the augend. Accordingly, the true or corrected count N.sub.0 will equal the sum of N+N'.
In the above noted copending U.S. application, Ser. No. 238,079, filed Mar. 27, 1972, methods and apparatus are disclosed for correcting coincidence count inaccuracies in a Coulter type of particle analyzer. In the crossreferenced patent application, it is stated that an error corrected particle pulse count can be obtained using the formula; EQU N.sub.0 = N.sub.R + KNR.sup.2 ( 1)
wherein N.sub.0 is the true or corrected count, N.sub.R is the raw count, and K is a constant which relates primarily to the physical parameters of the scanning elements of the apparatus. The copending application further states that two formulas such as formula (1) above can be simultaneously solved yielding an equation for N.sub.0 in terms of two related raw counts N.sub.1 and N.sub.2 only, thus eliminating K, a parameter which is difficult to ascertain. The equation for N.sub.0 , stated in terms of N.sub.1 and N.sub.2 only, is different for each way in which the two related counts N.sub.1 and N.sub.2 are developed. The two related counts are developed:
a. by passing a sample volume through different scanning apertures having a known difference in their critical volume relationship to obtain N.sub.1 and N.sub.2 ;
b. by passing a sample volume through different scanning apertures having the same critical volume and using one output as N.sub.1 and the sum of the outputs as N.sub.2 ;
c. by passing two different dilutions of known dilution relationship of the sample through a single scanning aperture to obtain N.sub.1 and N.sub.2 ;
d. by passing one sample through one scanning aperture to generate N.sub.1 and, by use of delaying and adding N.sub.1 to itself, to form N.sub.2.
An equation for N.sub.0 stated in terms of N.sub.1 and N.sub.2 only, is given in the copending application for each of these ways of developing N.sub.1 and N.sub.2.
The raw counts N.sub.1 and N.sub.2 developed using one of the above noted methods are accumulated in accumulators. When the counts have been accumulated, the total counts are used in one of the equations in order to yield the true or corrected count.
The disadvantage with the above noted method and apparatus is that the true or corrected count cannot be obtained until after both raw counts N.sub.1 and N.sub.2 have been completely accumulated. Furthermore, the equations used for obtaining the true count, and which are stated in terms of N.sub.1 and N.sub.2, are quite complicated. In order to quickly solve these equations the accumulated totals N.sub.1 and N.sub.2 must be fed to computational equipment such as a calculator or computer which can quickly compute the true or corrected count.